Acute graft-versus-host disease (aGVHD) after autologous progenitor cell transplantation has been associated with blood transfusion or cyclosporine. Mild aGVHD grades I-II, identified as autoaggression or engraftment syndrome, has recently been described in autologous progenitor transplantation. Here, we report the first case of pathologically documented grade IV aGVHD after autologous peripheral blood progenitor cell transplantation in a patient with breast cancer. The allogeneic origin was excluded by molecular techniques, and no cyclosporine or cytokines were administered.